www.nature.com/scientificreports OPEN New reconstruction of the Wiwaxia scleritome, with data from Chengjiang juveniles Received: 16 May 2015 1 2 1 Accepted: 09 September 2015 Zhifei Zhang , Martin R. Smith & Degan Shu Published: 07 October 2015 Wiwaxiids are a problematic group of scale-covered lophotrochozoans known from Cambrian Stages 3–5. Their imbricating dorsal scleritome of leaf-like scales has prompted comparison with various annelids and molluscs, and has been used as a template to reconstruct the articulation pattern of isolated Small Shelly Fossils. The first articulated specimens of Wiwaxia from the Cambrian Stage 3 Chengjiang Konservat-Lagerstätte show that the Wiwaxia scleritome comprised nine equivalent transverse rows associated with outgrowths of soft tissue, but did not possess a separate zone of anterior sclerites. This serial construction is fundamentally incompatible with the circumferential disposition of sclerites in early molluscs, but does closely resemble the armature of certain annelids. A deep homology with the annelid scleritome must be reconciled with Wiwaxia’s mollusc-like mouthparts and foot; together these point to a deep phylogenetic position, close to the common ancestor of annelids and molluscs. The distinctive mid-Cambrian organism Wiwaxia is best known for its stalked carbonaceous sclerites, which together comprise an imbricated dorsal scleritome. Articulated scleritomes have previously been reported from five localities spanning 15 million years1–6. The constitution of the Wiwaxia scleritome is remarkably conserved, notwithstanding species-level variety in sclerite proportions and orientation. An anterior zone of sclerites is followed by eight transverse rows across the body, with morphologies varying consistently according to location: ventrolateral sclerites are sickle-shaped; lower-lateral sclerites are oval; upper-lateral sclerites are rounded and symmetrical, and dorsal sclerites are asymmetric. In addition, mature specimens – those longer than a centimetre – exhibit twin series of erratically arranged dorsal spines. The anterior body region seemingly corresponds to a distinct zone of rounded sclerites4, although the exact relationship of these anterior sclerites to the transverse rows is unclear. The construction of the sclerites and scleritome represents important, if ambiguous, evidence with respect to the phylogenetic affiliation of Wiwaxia. The sclerites bear the distinctive signature of microvil- lar secretion – an internal microstructural fabric of long, narrow chambers – which assigns Wiwaxia to the lophotrochozoan total group7. (The persistent occurrence of these striations through the full length of intact sclerites distinguishes them from the parallel striations reported in certain ecdysozoan sclerites8,9). Beyond this, the interpretation of the scleritome is more ambiguous. Some authors10,11 have favoured an annelid analogue, but others12,13 have emphasized similarities with aculiferan molluscs – accommodating the conspicuously mollusc-like nature of the Wiwaxia foot (as observed in a small number of specimens) and feeding apparatus12,14. Here we report a new species of Wiwaxia based on articulated specimens from the Cambrian Stage 3 Chengjiang Lagerstätte. Post-mortem enrolment and soft tissue preservation in these fossils, in com- bination with critical W. corrugata material from the Burgess Shale, allows a timely re-evaluation of the Wiwaxia scleritome, and expounds the scleritome’s implications for the affinity of this confounding taxon. 1Early Life Institute, State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, 710069, P. R. China. 2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK. Correspondence and requests for materials should be addressed to Z.-F.Z. (email: [email protected] or [email protected]) SCIENTIFIC REPORTS | 5:14810 | DOI: 10.1038/srep14810 1 www.nature.com/scientificreports/ Figure 1. Wiwaxia papilio sp. nov. from Chengjiang. (a,b), ELI-W001, Holotype, part (horizontally mirrored) and counterpart, ventral view showing bundles of sclerites (b1–b9) associated with rust-coloured outgrowths of soft tissue (bold-face lettering), showing mouthparts (mp); (c), ELI-W004, ventral view, with anterior body curved up to display anterior sclerite zone; sclerites in anterior row express morphologies of ventrolateral (vl), lower lateral (ll), upper lateral (ul) and dorsal (d) sclerites; interpretative sketch in (f); (d), ELI-W003, anterior view showing mouthparts (mp); (e), ELI-W005, lateral view, illustrating anterior sclerite row and eight subsequent dorsal sclerites (1–9). Scale bars: 1 mm. Z. Zhang and M. Smith created the images. Material and Methods Five new Wiwaxia specimens, each comprising part and counterpart, have been collected from Chengjiang by the Early Life Institute working team, and deposited in the Early Life Institute and Department of Geology, Northwest University, Xi’an, China (Prefix: ELI). These complete, articulated specimens repre- sent organisms in various states of enrolment, preserved at various orientations to the plane of splitting. Sclerites and mouthparts are represented by regions with a deep purple to black colouration, and in the best cases correspond to an intact layer of carbon. Regions corresponding to soft tissue are coated with rust-coloured framboids, apparently arising through the oxidation of pyrite. Burgess Shale specimens of W. corrugata are deposited in the Smithsonian Institution National Museum of Natural History (NMNH), Washington DC, and the Royal Ontario Museum (ROM), Toronto, and represent unweathered carbonaceous compression fossils associated with diagenetic alu- minosilicate films15. Systematic Palaeontology. This published work and the nomenclatural acts it con- tains have been registered in Zoobank: http://zoobank.org/Referencesurn:lsid:zoobank. org:pub:1B5E0AE5-2FB2-4EFF-B35B-8293D919DEE8 Family Wiwaxiidae Walcott, 1911 (nom. corr. Howell, 1962) Genus Wiwaxia Walcott, 1911 Emended diagnosis. Ovoid body bearing nine transverse rows of ribbed carbonaceous sclerites, arranged in bundles and directed towards the posterior. Anteriormost sclerite row terminal. Sclerites comprising narrow root and wide blade, and incorporating narrow internal longitudinal chambers. Sclerite mor- phology varying consistently across each transverse row; medial sclerites rounded, ventro-lateral sclerites elongate and curved, usually with pointed tip. Dorsal surface of adults often with elongate spinose sclerites. Ventral surface comprising unarmoured ‘foot’. Toothed feeding apparatus comprising two to three rows of curved carbonaceous teeth arranged on tongue-like supporting apparatus. SCIENTIFIC REPORTS | 5:14810 | DOI: 10.1038/srep14810 2 www.nature.com/scientificreports/ Figure 2. Mouthparts of Wiwaxia. (a,b), ELI-W003, Wiwaxia papilio sp. nov., optical (a) and backscatter electron (b) images; (c,d) Wiwaxia corrugata from the Burgess Shale; (c), NMNH 277890; (d), NMNH 271947. Scale bars = 250 μ m. Z. Zhang and M. Smith created the images. Wiwaxia papilio sp. nov. LSID. urn:lsid:zoobank.org:act:E5A1A18C-D087-4FA1-AC60-A0CE65603E9B Derivation of name. Papilio (Latin), butterfly, reflecting the butterfly-like arrangement of the fans of sclerites. Holotype. ELI-W001 (Fig. 1a–b), an almost complete dorsoventral specimen preserving mouthparts and soft tissue. Paratypes. ELI-W002–ELI-W005 (Figs 1c–e, 2a,b). Stratigraphic setting. Specimens were collected from the yellowish-green to greyish-green mudstones of the Chengjiang Lagerstätte at the Jianshan Section in Haikou, Kunming. Other taxa recovered from this site include the early agnathan Haikouichthys16 and the echinoderm-like vetulocystids17. Diagnosis. Single order of widely spaced sclerite ribs (4–6 ribs on sclerites 500–1000 μ m in length). Non-ventrolateral sclerites long and wide relative to body length (Fig. 3). Remarks. The examined material resembles juvenile specimens of Wiwaxia corrugata in terms of its overall body size, the form of its mouthparts, the relatively large size of dorsal sclerites, the broad yet short ventrolateral sclerites, and the absence of dorsal spines. Adult specimens are conceivably repre- sented by the larger isolated sclerites that have also been reported from Chengjiang18 (Fig. 3), though this material is difficult to exclude from other Wiwaxia species. W. papilio sp. nov. is distinguished from W. corrugata based on the low number ribs on its sclerites; detailed comparison with other species is hampered by the shortage of comparative material1,5. Description. The articulated specimens of W. papilio sp. nov. (Figs 1 and 4) are 5–8 mm long, and exhibit the arrangement of sclerites typical of juvenile Wiwaxia specimens: an anterior region of sclerites fol- lowed by eight further transverse rows, with no dorsal spines. Ventrolateral sclerites are siculate, whereas other sclerites are rounded and about twice as long as broad. (The limited preservation of the dorsal surface precludes a detailed description of the dorsal most sclerites.) Each ventro-lateral sclerite fully SCIENTIFIC REPORTS | 5:14810 | DOI: 10.1038/srep14810 3 www.nature.com/scientificreports/ Figure 3. Summary of sclerite measurements in Wiwaxia species. c W. corrugata; f W. foliosa; p, W. papilio sp. nov.; + , isolated Chengjiang sclerites. Dashed lines denote 95% the confidence envelope of regression lines. Within each panel, regression line gradients are not significantly different. Panel legends report significant